In electrical engineering, a converter represents a device intended for changing the parameters of electricity. The basic parameters of electricity are the size of voltage, current, as well as the frequency in alternating power supplies. Based on the physical laws, it implies that the energy conversion efficiency is always less than 100% and every converter has an energy loss. Electricity converters mostly operate with very good efficiency with losses around 5-10% around the operating point however when the device is unloaded the relative losses tend to be higher. The most commonly used converter is a transformer, a device that enables changes to the size of AC voltage. In the past, electromechanical rotary converters were used for changing the size of DC voltage, which consisted of a DV electric motor and generator arranged on a common shaft.
Today, the most common converters are semiconductor converters for changing voltage or frequency. Depending on its application the converter may be: a rectifier; an inverter, which inverts the AC into DC power frequency converter, which converts the frequency of alternating voltage and current.
Recently active rectifiers, which also allow the recovery (a reverse flow of power—and returning the energy into the AC power network) have been used.
Now semiconductor converters can be found in almost every electrical device, either switching power supplies for computers or other consumer electronics; frequency converters in compact fluorescent bulbs, rechargeable batteries, in microwave ovens or induction heating cookers. One of the most important applications of semiconductor converters are in the regulated electric drives, allowing a significant increase in efficiency, dynamics, stability, accuracy and value of the usage of the drive.
Voltage multiplier is a voltage converter consisting of diodes and capacitors, which converts AC voltage into DC voltage several times higher than its value. The circuit is connected in such way that one half cycle of alternating current charges the capacitors parallelly and the second (opposite) half cycle uncharges them in series connection. For one pair of diode and capacitor twice the input voltage is formed at the output. It is possible to repeat this basic circuit in a cascade and thus create an output with much higher voltage. The voltage multiplier has been used as a cheap and small substitute for a transformer where there is low consumption of current with high voltage. An example is an old electric insect trap, in which the mains voltage (220 V) produced several thousands of volts used for burning insects. Also some TVs use voltage multiplier as a source of preload for the screen. This circuit is also used in physics and wherever it is needed to easily produce high DC voltage, because it provides preferable spatial distribution of voltage. Ideally, at one level the voltage corresponds to a double amplitude of the source voltage, from which it is powered. A device working on this principle is called a cascade generator.
In general a circuit working on this principle is called a charge pump and is used in electronics, wherever a cheap power supply with low power is needed. For example, level converters for standard serial RS232, such as MAX232 circuit.
They had been used in receivers without transformers with mains voltage of 110 to 125 V where the AC voltage was not sufficient to reach high enough DC voltage.
Rectifier is an electric device which converts AC electrical power to DC electrical power. As the electrical circuits need DC current to function and AC current is used for distribution of electricity, the rectifier is usually a part of electrical devices and consumer electronics devices, powered from the mains supply. Rectifiers are often used in power systems of electric traction vehicles (e.g. propulsion of locomotives, trams, trolley buses or subway cars). Today semiconductor rectifiers based on silicon are almost exclusively used which have almost entirely replaced other devices. New devices based on silicone carbide may emerge with the advantage of operating at higher temperature.
A complex one-port Resonant circuit is formed by parallel or series connections of capacitor and coil. At a certain frequency, the resonant frequency, the capacitive and inductive reactance are balanced and the resonant circuit behaves like an active resistor at this frequency. The state of the circuit, which occurs at the resonant frequency, is called resonance. It is a phenomenon in which at the certain frequency the RLC circuit increases the current in a series circuit or substantially increases the voltage in a parallel circuit. Series resonant circuit has a resonant frequency at the lowest impedance. Parallel resonant circuit has a resonant frequency at the highest impedance. The circuit at this frequency has only an effective resistance.
We have two electric components which are able to gather energy:                capacitor—gather electrostatic energy        coil—gather magnetic energy        
We connect both components into an electric CL circuit, charge the capacitor and then couple it with a coil. The capacitor is short-circuited through the coil, starts to discharge, however, the current does not increase sharply (the coil induces counter-voltage), the capacitor gets discharged (its energy decreases), the increasing electric current flows through the coil and generates increasing magnetic field (and also increasing the energy comprised in the coil), the capacitor discharges (the voltage decreases to zero) and all the energy is transferred to the coil. There is no other electric charge on the capacitor plates which would incite the current however the current does not drop to zero because the coil maintains the current status (current flow) and induces current flowing in the same direction as it did at the beginning, so that the capacitor charges to the opposite polarity than at the beginning (the energy is transferred from the coil back to the capacitor). The current gradually decreases and the voltage on the capacitor increases, the capacitor is charged to the same voltage but opposite polarity (energy is transferred from the coil back to the capacitor) and this is repeated again in the opposite direction.
Bifilar coil in electrical engineering is defined as a coil made of a double conductor (a pair of concurrently winded wires). If we connect both conductors on one end, the current in the neighbouring conductors flows in the opposite direction. The resulting magnetic fields counteract each other and their effects cancel each other. Such coil is used in production of wire wound resistors with very low parasitic inductance.
When we use the both conductors as a separate transformer coils, a transformer with exceptionally low leakage inductance is obtained. Bifilar as well as multiple-winding transformers stand out with their particularly good pulse transmission characteristics. These characteristics are useful for example when operating pairs of switching transistors. Conductors of such transformer are wound parallelly and eventually mutually twisted. A drawback is, of course, the increase of capacity of such closely related coils. Bifilar coils are nowadays used as choking coils for various voltage filters and compensating members. Geometric characteristics determine the characteristics of each coil. Surface of the coil core affects the transmitted power, wire diameter and its material affects the size of resistance as well as the flowing current, and the number of turns affects the size of the output voltage.
Today various forms of electric heating are used. Direct conversion of electrical energy into heat is achieved using the resistance connected in the electric circuit. Heating elements with resistance powered by AC voltage and current are used as loads in these devices. This principle is used in thermal resistance devices, e.g. electric boilers which are usually powered from the mains with AC voltage and current with the frequency of 50 Hz. Also appearing are installations using DC voltage and current which are considered to be more effective. These are also more economical in terms of thermal resistance because the constant vibrations and temperature changes do not occur.
In installation today we have met the demand for increased heating capacity of the existing as well as newly installed and produced heating systems. In addition we have demonstrated a reduction in heating times and reaching higher temperatures for a comparative and constant input. In certain technological processes this has lead to increasing the efficiency of the production processes. In commercial and domestic properties it has lead to providing a higher thermal comfort despite the existence of insufficiently dimensioned heating through shortening the time of heating and increasing the temperature in the interior spaces whilst reducing the power consumed.